A method of recovering hydrocarbon liquid and gas fluids from an underground tar sand or heavy oil reservoir is known which comprises (a) drilling and completing a pair of wells, which pair comprises an injection well terminating in the reservoir and a production well terminating in the reservoir below the injection well; and (b) creating a permeable zone between the injection well and the production well.
After having created permeable zones between the injection well and the production well steam injection through the production well is stopped and steam is only injected through the injection well while fluids are produced through the production well.
It is believed that the injected steam forms in the reservoir a steam-containing, heated zone around and above the injection well and that fluids (throughout) are mobilized in the heated reservoir and drain by gravity through the heated zone to the production well which is located below the injection well. Therefore this method is referred to as steam assisted gravity drainage.
It is an object of the present invention to improve the known method.